1. Field of the Invention
The present invention relates to portable power sources, power sources for electric automobiles, fuel cells for use in household cogeneration systems and the like, and in particular, to a polymer electrolyte fuel cell employing a polymer electrolyte.
2. Description of Related Technology
In fuel cells employing polymer electrolytes, a fuel gas containing hydrogen is electrochemically reacted with an oxidizing gas containing oxygen, such as air, to simultaneously generate power and heat. This fuel cell is basically comprised of a polymer electrolyte membrane selectively transferring hydrogen ions and a pair of electrodes formed on either side of the polymer electrolyte membrane, that is, an anode and a cathode. The electrodes are comprised of a catalyst layer, formed on the surface of the polymer electrolyte membrane, that is comprised chiefly of carbon powder supported on a platinum group metal catalyst, and a gas diffusion layer, having both gas permeability and electron-conducting capability, formed on the outer surface of the catalyst layer.
To prevent oxidizing gas and fuel gas fed to the electrodes from leaking to the exterior and to prevent mixing of the two gases, the electrodes are formed on the two surfaces of portions outside the rim portion of the polymer electrolyte membrane and gas seals and gaskets are positioned in a manner surrounding the electrodes on the rim portions of the polymer electrolyte membrane. These gas seals and gaskets are integrated and preassembled with the electrodes and polymer electrolyte membrane. This integrated and preassembly combination is hereinafter referred to as a membrane electrode assembly (“MEA”). The MEA is mechanically secured and electrically conductive separators for electrically connecting adjacent MEAs in series are provided on both sides thereof. Gas passages for feeding reaction gas to the electrode surface and carrying off water that is generated and excess gas are formed on the portions where the separators contact the MEA. The gas passages can be provided separately from the separators, but the method of forming grooves serving as gas passages on the outer surface of the separator is generally adopted.
The supplying of reaction gas to the gas passages and the discharging of water produced and reaction gas from the gas passages are conducted by providing a through-hole known as a manifold hole in the separator, connecting the gas passage inlet and outlet to the manifold hole, and distributing reaction gas to the various gas passages through the manifold hole. Since the fuel cell generates heat during operation, maintaining the cell at proper temperature requires cooling with a cooling fluid, such as cooling water, or the like. Normally, a cooling member through which cooling water flows is provided for every one to three cells. The MEA, separator, and cooling member are stacked in alternating fashion to achieve 10 to 200 cell layers, after which they are sandwiched between terminal plates through current collecting plates and insulating plates, and then secured at each end with fastening rods into a common stacked cell structure. While not wishing to be bound to a specific construction of a cell stack, a typical construction can be found in U.S. Published Application US 2002/01456601 and U.S. Pat. No. 6,413,664, both herein incorporated by reference.
Perfluorosulfonic acid-based materials have come to be used in the polymer electrolyte membranes of such cells. It is normally necessary to moisten the fuel gas and oxidizing gas and feed them to the cell for the polymer electrolyte membrane to develop ion conductivity in a moisture-comprising state. On the cathode side, water is generated in some reactions. Thus, when a gas that has been moistened to increase the dew point to above the working temperature of the cell is supplied, there is sometimes a problem in that condensation forms in the gas passages in the cell and within the electrodes and water blockage occurs, rendering cell performance unstable and decreasing performance. This phenomenon of decreased cell performance and unstable operation due to excessive wetting is generally called “flooding”. Additionally, when the fuel cell is being used in a power generating system, it is necessary to systemize the wetting of feed gas and the like. To simplify the system and enhance system efficiency, it is desirable to at least slightly reduce the dew point of the wetted gas that is supplied.
From the perspectives of preventing flooding, enhancing system efficiency, and simplifying the system set forth above, the feed gas is usually moistened so that its dew point is slightly lower than the cell temperature before being supplied.
However, it is also necessary to increase the ion conductivity of the polymer electrolyte membrane to increase the performance of the cell. Thus, it is desirable to moisten the fuel gas and feed it at a relative humidity of near 100 percent, or even at 100 percent humidity and above. Further, from the perspective of the durability of the polymer electrolyte membrane, as well, the supplying of feed gas with a high moisture content is known to be desirable. However, when supplying moist gas at, or close to 100 percent relative humidity, the above-described flooding becomes a problem. That is, in operation of prior cells, the humidity of the feed gas cannot be adjusted accurately and precisely enough to inevitably prevent flooding.
A method of increasing the flow rate of feed gas through the separator passage portion to blow out water that has condensed is known to be an effective way to remove condensation to avoid flooding. However, it then becomes necessary to feed the gas at high pressure to increase the feed gas flow rate in order to blow out the condensed water, requiring an extreme increase in the auxiliary power of the blower or compressor supplying the gas when using the cells in a systemized environment thereby tending to result in deterioration of system efficiency. Further, when flooding occurs on the anode side, flow of fuel gas tends to be blocked or diminished, which ends up being fatal to the fuel cell. This is because load current is forcefully removed in a state where the flow of fuel gas is inadequate, and the carbon supporting the catalyst of the anode ends up reacting with water in the atmosphere to produce electrons and protons in a state without fuel. As a result, dissolution of carbon in the catalyst layer permanently damages the catalyst layer of the anode.
Furthermore, in systems in which stacked cells are mounted, commercial considerations dictate that the cell be able to operate not only under rated output conditions, but also operate under low loads when output is reduced based on power demand. Maintaining efficiency under low load operation requires that the use rates of fuel gas and oxidizing gas be made identical to rated operation conditions. That is, when the load is reduced by ½ relative to rated operation, for example, if the flow rates of the fuel gas and oxidizing gas are not decreased by about ½, excess fuel gas and oxidizing gas are consumed, causing power generation efficiency to drop. However, when the gas use rate is preset and low load operation is conducted, there are problems in that the gas flow rate in the gas passages decreases, condensation water and generated water cannot be discharged from the separator, the above-described flooding occurs, cell performance decreases, and performance becomes unstable.
It is also known that condensed water and generated water collect in portions of the gas passage where the flow runs against gravity if such portions are present, tending to cause flooding. As a countermeasure, methods of making the oxidizing gas or fuel gas flow in directions that do not run against gravity have been proposed (Japanese Patent Application Publication Nos. Hei 11-233126 and 2001-068131, both incorporated herein by reference). Based on these methods, the oxidizing gas or fuel gas is made to flow in directions that do not run counter to gravity, thereby permitting smooth discharge of condensed water and generated water and inhibiting flooding. However, this proposal does not address the drying out of the polymer electrolyte membrane necessary to develop ion conductivity.